Investigation into the rapid compression of semi-solid alloy slugs

The shaping of metals by thixoforming relies on the unusual flow behaviour of slurries containing non-dendritic solid, which was first studied at MIT in the early 1970s. It was discovered that if an alloy is uniformly stirred during freezing, discrete rounded particles of solid are formed rather than the normal dendrites, and the slurry is highly fluid (10 P or 1 Pa s, like machine oil) up to at least 50% solid. Furthermore, these slurries were found to be thixotropic, in that their apparent viscosity is dependent on shear rate and time; the more vigorously they are stirred, the more fluid they become. However, on standing without stirring they begin to stiffen and become more viscous (107 P or 106 Pa s, rather like stiff butter). Stirring again will cause a return to relatively low viscosity. It has also been shown that the alloy microstructure has an important and sensitive effect on fluidity. Ideally, rounded solid particles, without any entrapped liquid, are needed to obtain the highest fluidity. Under normal thixoforming conditions, the semi-solid slugs experience rapid compression before flowing into the die cavity. During a very short time interval, they experience intense shear thinning and their flow behaviour changes from one resembling a solid able to support its own shape, to that of a viscous liquid. Here, a technique of rapid compression testing is outlined, carried out under conditions similar to normal thixoforming, to assess the semi-solid flow behaviour.